This patent disclosure relates generally to apparatus and methods for cleaning contaminants from a substrate and, more particularly, to apparatus and methods for cleaning contaminants from a substrate using an ultra-clean, dry fluid stream.
Wafer cleanliness is recognized as a critical parameter in solid state electronic device manufacturing processes. And as the spatial scale of features on semiconductor devices decreases, the threshold size of particles that contribute to manufacturing defects also decreases, thereby placing higher demands on cleaning processes to remove the smaller particles. Indeed, particles as small as one micrometer, or smaller, can pose deleterious effects in current-day semiconductor fabrication. Further, the increased demands on cleaning method performance due to decreasing feature size is compounded by increasing wafer sizes because of the rise in the number of features on a wafer, posing more opportunities for cleanliness-related defects.
Some conventional approaches are known for removing particles and residue from substrates in the semiconductor fabrication industry and other industries with similar sensitivities to cleanliness. These approaches can be categorized as either physical cleaning techniques, such as brush scrubbing, dry argon ice cleaning, carbon dioxide (CO2) aerosol techniques, or chemical cleaning techniques, such as plasma etching, or wet etching (with or without ultrasonic or megasonic agitation).
Physical aerosol jet cleaning has been demonstrated for dry removal of submicron contaminant particles. Aerosol particles can be formed in a gas stream by solidification of liquid droplets or the gaseous medium during rapid cooling. When the solid aerosol particle collides with a contaminant, the resulting momentum transfer generates a force on the contaminant that may overcome an adhesion force between the particle and a substrate, and thereby remove the particle or residue from the surface of the substrate. The CO2 aerosol cleaning technique has been used for a variety of surface cleaning applications such as silicon (Si) wafers, photomasks, MEMS devices, packaging fabrication, imaging devices metal lift-off, ion implanted photoresist stripping, disk drives, flat panel displays, and post-dicing for three-dimensional stacked integrated circuits.
Plasma cleaning, on the other hand, is based on generating reactive species to eliminate organic contaminants by chemically converting the organic contaminants into volatile gaseous products. The reactive species used for plasma cleaning may include, for example, radicals or excited atoms created from oxygen, hydrogen, combinations thereof, or other similarly suited reactive species known to persons with ordinary skill in the art.
In the photomask industry, for example, some important issues are yield loss, cost, cycle time of mask technology, mask supply, and mask lifetime. There are numerous yield loss mechanisms associated with mask technology, including: excessive quantity of lithography patterning defects, hard defects (i.e., un-repairable defects soft defects (i.e., particle defects), particle defects after pellicle mounting, uniformity of the Critical Dimensions (CD), pellicle mounting error, and errors related to Optical Proximity Corrections (OPC). In some cases, the major process-related yield loss mechanisms may be hard and soft defects up to 56-67% of the total number of defects depending on the type of mask. Some reasons for performing mask maintenance may include soft defects (26%) and hard defects (10%). In other cases, the need for mask maintenance has been motivated by non-removable particles, ranging up to 34% of total maintenance and service calls. In summary, there exists a need to enhance the current cleaning technologies to enable removal of all possible particles from mask surfaces to meet increasingly stringent specifications.
Photomasks are utilized to repeatedly print fine features on wafers for high volume production. Mask lifetime may be reduced by growth of an organic and/or inorganic layer of defects (also called haze), electro-static discharge (ESD), non-removable particles, transmission loss, reflectivity loss, phase change, change in printed CD uniformity, and the like. These defects may be introduced from cleaning tools in addition to other process tools that may be used in the mask fabrication process or in wafer fabrication.
Conventional solvent cleaning techniques may result in sufficient degradation of the mask to limit mask life. Indeed, photomask life may be evaluated according to stringent specifications, and even a single parameter falling outside the specification may be sufficient to end a photomask's useful life. Foreign material and stains are known as soft defects on masks that require cleaning. Next generation lithography (NGL) masks, including Extreme Ultraviolet (EUV) and Nano-imprint lithography (NIL) masks, may be subject to life-limiting degradation issues caused by current cleaning technologies, and may also be affected by multiple types of contamination.
In optical mask technology, the active area may be protected from soft defects by adhering a pellicle to the mask. In general, three types of pellicle adhesive are available, including organosilcone, styrene-ethylene-butylene-styrene (SEBS), and methacrylate copolymer, for example. At times, the pellicle must be removed from the mask to perform repairs and perform cleaning to cure mask defects. Adhesive or glue residue “tracks” may remain following pellicle removal, and may require removal before a new pellicle can be applied to the mask. Removal of pellicle adhesive residues and subsequent reticle cleaning can be a costly and time-consuming operation. Not only are the wet chemicals costly, but safety and environmental issues may add further costs with respect to reticle cost-of-ownership.
Additionally, wet chemical processes have the potential to cause damage including critical dimension changes due to chemical attack, particularly for advanced phase-shifting reticles. The objective of reticle cleaning and pellicle reinstallation is to return the reticle to the wafer fabrication process unchanged with respect to its original performance. Ideally, a cleaning technique will, remove all pellicle adhesive residues, soft defects, organic and inorganic particles, with low cost and time requirements, and without damaging the mask.
For EUV mask technology the main sources of contamination are oxidation and the buildup of carbon contamination layers. Further, water may act as an oxidizer, and may etch surfaces such as silicon. In addition, carbon contamination (in film or particle form), may result from photon and/or electron enhanced dissociation of residual carbon-containing molecules in EUV exposure tools. In turn, the critical dimensions and reflectivity changes may influence the printability of the ask. Current solutions include reducing the contamination rate and/or cleaning the mask surfaces. A heavy metal capping layer that forms a stable oxide layer and prolongs the life of the mirror, albeit with a small reflectivity loss, may hinder cleaning EUV mask surfaces. In some cases, the capping layer may not prevent a build-up of carbon contamination, thereby making carbon build-up the main surface contamination process. Indeed, even under relatively good vacuum conditions, carbon contamination has been observed. Further, the high absorption of EUV radiation by carbon may compound the optical losses therethrough. Ideally, a cleaning technique is desired that will remove contaminants from EUV masks including soft defects, organic particles, and inorganic particles, without damaging the mask.
The semiconductor industry has developed cleaning and surface preparation techniques, including particle removal, residue removal, surface cleaning, and etching to promote production. However, the need for cleaning and surface preparation techniques extends beyond the semiconductor industry. Indeed, other fields such as biology, medicine (implants and equipment), aerospace, imaging, automobiles, pharmaceuticals, and the like, benefit from surface cleaning and preparation processes. In general, methods and apparatus for removing organic and inorganic defects including particle contaminants, chemical residues, pellicle adhesive residue, photoresist residue, carbon contamination, contamination adders from process tools, and the like, from substrates, as well as methods and apparatus for surface activation/conditioning may benefit a variety of industries such as lithography (optical and Extreme Ultra Violet (EUV) masks), semiconductors, compound semiconductors, MEMS, disk drives, imaging devices, LED/flat panel displays, photovoltaics, and other similar industries know to persons having ordinary skill in the art. In addition, aspects of the present disclosure may also be advantageously combined with other dry or wet cleaning techniques including, but not limited to, wet cleaning, laser shock cleaning, and frequency-assisted cleaning.
Some laboratories have investigated plasma cleaning processes using vacuum-based technology to remove surface contaminants from silicon substrates. Techniques including radio frequency (RF) remote plasma (also known as capacitive-coupled RF plasma) and electron cyclotron resonance (ECR) plasma may be used with carrier gases such as argon or helium, and gases such as oxygen or hydrogen, to generate excited atoms, molecules, or ions for etching processes. However, some of these conventional approaches may require expensive vacuum provisions, which may also contribute additional contaminants to the substrate as a result of pump down and venting, and which may damage the substrate due to accelerated ions or charged species directed onto the substrate. In addition, some of these conventional plasma approaches may not conveniently effect local or site-specific cleaning.
Conventional aerosol CO2 cleaning is based on momentum transfer between the fast-moving snow aerosol generated through a nozzle and the surface contaminant. In situations where adhesion between the residue and surface is strong, such as organic contaminants, for example, large CO2 snow with sufficient velocity to remove the residue may also cause damage to fine structures nearby. Further, the residue dislodged by the CO2 snow may need to be carried away from the target area by a fluid flow and be filtered such that cleanliness of the chamber is preserved.
Convert tonal cleaning methods using atmospheric plasma sources may pose some disadvantages. First, the localized plasma source such as an atmospheric plasma may not remove inorganic contaminants. Second, there are few designs available for an atmospheric plasma etcher. One review article by Schütze et al, (“The Atmospheric-Pressure Plasma Jet: A Review and Comparison to Other Plasma Sources,” IEEE Transactions on Plasma Science, Vol. 26, No. 6, (December 1998)), has explained in detail some designs of atmospheric pressure plasma. However, there are many modifications needed before it could be considered suitable for organic residue removal. For example, the literature has failed to suggest suitable materials to be used for the source chamber. Because of the harsh environment of the plasma, contamination may be generated from interaction of the plasma with the source chamber wall containing it. Anodized aluminum has been proposed, however, the oxide formed on aluminum in this process may not be smooth, could be non-uniform in thickness, may not exhibit sufficient purity with appropriate stoichiometry, and may not meet the exacting cleanliness requirements for a semiconductor fabrication environment.
A quartz tube with thickness of the order of millimeter has been proposed. Because the thermal conductivity of quartz is more than an order of magnitude less that of metals such as aluminum or stainless steel, reaching thermal steady state and maintaining desired temperature may be drawbacks of using quartz. In addition, quartz may not readily accommodate tailoring of the shape of the plasma container to adjust fluid characteristics of the gaseous jet issuing therefrom.
Species issuing from the source may not be collimated, and therefore, the jet may spread laterally. As a result, structures neighboring a surface intended to be cleaned could be exposed to the species emitting from the plasma and corrode them. Indeed, in some applications, primary features on the wafer or mask may not tolerate plasma exposure.
Further, conventional plasma source cleaning apparatus designs may not simultaneously accommodate CO2 snow for physical removal of residues and provide vacuum pressure conditions sufficient to promote plasma operation.